. 24
( 32 .)


6. Show that if R2 acts on itself by translation then, with respect to the standard area
form „¦ = dx§dy, this action is Hamiltonian but not Poisson.

7. Verify the claim made in the proof of Theorem 1 that the following identity holds for
all a ∈ G, all y ∈ g, and all m ∈ M:

»a »— (y)(m) = »— Ad(a)y (a · m).

8. Here are a few mechanical exercises that turn out to be useful in calculations:
(i) Show that if »i : G — Mi ’ Mi for i = 1, 2 are Poisson actions on symplectic
manifolds (Mi , „¦i ) with corresponding momentum mappings µi : Mi ’ g— , then
the induced product action of G on M = M1 — M2 (where M is endowed with the
product symplectic structure) is also Poisson, with momentum mapping µ : M ’ g—
given by µ = µ1 —¦π1 + µ2 —¦π2 , where πi : M ’ Mi is the projection onto the i-th factor.
(ii) Show that if » : G—M ’ M is a Poisson action of a connected group G on a symplectic
manifold (M, „¦) with equivariant momentum mapping µ : M ’ g— and H ‚ G is
a (connected) Lie subgroup, then the restricted action of H on M is also Poisson
and the associated momentum mapping is the composition of µ with the natural
mapping g— ’ h— induced by the inclusion h ’ g.

E.7.2 126
(iii) Let (V, „¦) be a symplectic vector space and let G = Sp(V, „¦) ( Sp(n, R) where the
dimension of V is 2n). Show that the natural action of G on V is Poisson, with
momentum mapping µ(x) = ’ 1 x — (x „¦) . (Use the identi¬cation of g = sp(V, „¦)

with g de¬ned by the nondegenerate quadratic form a, b = tr(ab).) Show that the
mapping S 2 (V ) ’ sp(V, „¦) de¬ned on decomposables by
x—¦y ’ ’ x — (y „¦) + y — (x „¦)
is an isomorphism of Sp(V, „¦)-representations. Using this isomorphism, we can inter-
pret the momentum mapping as the quadratic mapping µ : V ’ S 2 (V ) de¬ned by
the rule µ(x) = 1 x2 . What are the clean values of µ? (There are no regular values.)
˜ 2
Let Mk be the product of k copies of V and let G act ˜diagonally™ on Mk . Discuss the
clean values and regular values (if any) of µk : Mk ’ g— . What can you say about the
corresponding symplectic quotients? (It may help to note that the group O(k) acts
on Mk in such a way that it commutes with the diagonal action and the corresponding
momentum mapping.)

9. The Shifting Trick. It turns out that reduction at a general ξ ∈ g can be reduced
to reduction at 0 ∈ g. Here is how this can be done: Suppose that » : G — M ’ M is
a Poisson action on the symplectic manifold (M, „¦), that µ : M ’ g— is a corresponding
momentum mapping, and that ξ is an element of g— . Let M ξ , „¦ξ be the symplectic
product of (M, „¦) with (G·ξ, „¦ξ ) and let µξ : M ξ ’ g— be the corresponding combined
momentum mapping. (Thus, by the computation for coadjoint orbits done in the text,
µξ (m, a·ξ) = µ(m) ’ a·ξ.)
(i) Show that 0 ∈ g— is a clean value for µξ if and only if ξ is a clean value for µ.
Assume for the rest of the problem that ξ is a clean value of µ.
(ii) There is a natural identi¬cation of the G-orbits in (µξ )’1 (0) ‚ M ξ with the Gξ -
orbits in µ’1 (ξ) and that there is a smooth structure on G\(µξ )’1 (0) for which the
map π0 : (µξ )’1 (0) ’ G\(µξ )’1 (0) is a smooth submersion if and only if there is

a smooth structure on Gξ \µ’1 (ξ) for which the map πξ : µ’1 (ξ) ’ Gξ \µ’1 (ξ) is a
smooth submersion. In this case, the natural identi¬cation of the two quotient spaces
is a di¬eomorphism.
(iii) This natural identi¬cation is a symplectomorphism of (M ξ )0 , („¦ξ )0 with (Mξ , „¦ξ ).
This shifting trick will be useful when we discuss K¨hler reduction in the next Lecture.

10. Matrix Calculations. The purpose of this exercise is to let you get some practice
in a case where everything can be written out in coordinates.
Let G = GL(n, R) and let Q: gl(n, R) ’ R be a non-degenerate quadratic form.
Show that if we use the inclusion mapping x: GL(n, R) ’ Mn—n as a coordinate chart,
then, in the associated canonical coordinates (x, p), the Lagrangian L takes the form
L = 1 x’1 p, x’1 p Q . Show also that ωL = x’1 p, x’1 dx Q .
Now compute the expression for the momentum mapping µ and the Euler-Lagrange
equations for motion under the Lagrangian L. Show directly that µ is constant on the
solutions of the Euler-Lagrange equations.

E.7.3 127
Suppose that Q is Ad-invariant, i.e., Q Ad(g)(x) = Q(x) for all g ∈ G and x ∈ g.
Show that the constancy of µ is equivalent to the assertion that p x’1 is constant on the
solutions of the Euler-Lagrange equations. Show that, in this case, the L-critical curves in
G are just the curves γ(t) = γ0 etv where γ0 ∈ G and v ∈ g are arbitrary.
Finally, repeat all of these constructions for the general Lie group G, translating
everything into invariant notation (as opposed to matrix notation).

11. Euler™s Equation. Look back over the example given in the Lecture of left-invariant
metrics on Lie groups. Suppose that γ: R ’ G is an L-critical curve. De¬ne ξ(t) =
„Q ω(γ(t)) . Thus, ξ: R ’ g— . Show that the image of ξ lies on a single coadjoint orbit.

Moreover, show that ξ satis¬es Euler™s Equation:

ξ + ad— „Q (ξ) (ξ) = 0.

The reason Euler™s Equation is so remarkable is that it only involves “half of the
variables” of the curve γ in T G.

Once a solution to Euler™s Equation is found, the equation for ¬nding the original
curve γ is just γ = Lγ „Q (ξ) , which is a Lie equation for γ and hence is amenable to

Lie™s method of reduction.
Actually more is true. Show that, if we set ξ(0) = ξ0 , then the equation Ad— (γ)(ξ) = ξ0
determines the solution γ of the Lie equation with initial condition γ(0) = e up to right
multiplication by a curve in the stabilizer subgroup Gξ0 . Thus, we are reduced to solving
a Lie equation for a curve in Gξ0 . (It may be of some interest to recall that the stabilizer
of the generic element · ∈ g— is an abelian group. Of course, for such ·, the corresponding
Lie equation can be solved by quadratures.)

12. Project: Analysis of the Rigid Body in R3 . Go back to the example of the
motion of a rigid body in R3 presented in Lecture 4. Use the information provided in
the previous two Exercises to show that the equations of motion for a free rigid body
are integrable by quadratures. You will want to ¬rst compute the coadjoint action and
describe the coadjoint orbits and their stabilizers.

13. Verify that, under the hypotheses of Theorem 2, the dimension of the reduced space
Mξ is given by the formula

dim Mξ = dim M ’ dim G ’ dim Gξ + 2 dim Gm

where Gm is the stabilizer of any m ∈ µ’1 (ξ).
(Hint: Show that for any m ∈ µ’1 (ξ), we have

dim Tm µ’1 (ξ) + dim Tm G · m = dim M

and then do some arithmetic.)

E.7.4 128
14. In the reduction process, what is the relationship between Mξ and MAd— (g)(ξ) ?

15. Suppose that »: G — M ’ M is a Poisson action and that Y is a symplectic vector
¬eld on M that is G-invariant. Then according to Proposition 1, Y is tangent to each
of the submanifolds µ’1 (ξ) (when ξ is a clean value of µ). Show that, when the sym-
plectic quotient Mξ exists, then there exists a unique vector ¬eld Yξ on Mξ that satis¬es
Yξ πξ (m) = πξ Y (m) . Show also that Yξ is symplectic. Finally show that, given an
integral curve γ: R ’ Mξ of Yξ , then the problem of lifting this to an integral curve of Y
is reducible by “¬nite” operations to solving a Lie equation for Gξ .
This procedure is extremely helpful for two reasons: First, since Mξ is generally quite
a bit smaller than M, it should, in principle, be easier to ¬nd integral curves of Yξ than
integral curves of Y . For example, if Mξ is two dimensional, then Yξ can be integrated
by quadratures (Why?). Second, it very frequently happens that Gξ is a solvable group.
As we have already seen, when this happens the “lifting problem” can be integrated by (a
sequence of) quadratures.

E.7.5 129
Lecture 8:

Recent Applications of Reduction

In this Lecture, we will see some examples of symplectic reduction and its generaliza-
tions in somewhat non-classical settings.
In many cases, we will be concerned with extra structure on M that can be carried
along in the reduction process to produce extra structure on Mξ . Often this extra structure
takes the form of a Riemannian metric with special holonomy, so we begin with a short
review of this topic.
Riemannian Holonomy. Let M n be a connected and simply connected n-manifold,
and let g be a Riemannian metric on M. Associated to g is the notion of parallel transport
along curves. Thus, for each (piecewise C 1) curve γ: [0, 1] ’ M, there is associated a linear
mapping Pγ : Tγ(0) M ’ Tγ(1) M, called parallel transport along γ, which is an isometry of
vector spaces and which satis¬es the conditions Pγ = Pγ and Pγ2 γ1 = Pγ2 —¦ Pγ1 where γ ¯
is the path de¬ned by γ (t) = γ(1 ’ t) and γ2 γ1 is de¬ned only when γ1 (1) = γ2 (0) and, in
this case, is given by the formula

for 0 ¤ t ¤ 1 ,
γ1 (2t) 2
γ2 γ1 (t) =
γ2 (2t ’ 1) for 2 ¤ t ¤ 1.

These properties imply that, for any x ∈ M, the set of linear transformations of the form Pγ
where γ(0) = γ(1) = x is a subgroup Hx ‚ O(Tx M) and that, for any other point y ∈ M,
we have Hy = Pγ Hx Pγ where γ: [0, 1] ’ M satis¬es γ(0) = x and γ(1) = y. Because we
are assuming that M is simply connected, it is easy to show that Hx is actually connected
and hence is a subgroup of SO(Tx M).
Elie Cartan was the ¬rst to de¬ne and study Hx . He called it the holonomy of g at x.
He assumed that Hx was always a closed Lie subgroup of SO(Tx M), a result that was only
later proved by Borel and Lichnerowitz (see [KN]).
Georges de Rham, a student of Cartan, proved that, if there is a splitting Tx M =
V1 • V2 that remains invariant under all the action of Hx , then, in fact, the metric g is
locally a product metric in the following sense: The metric g can be written as a sum of the
form g = g1 + g2 in such a way that, for every point y ∈ M there exists a neighborhood U
of y, a coordinate chart (x1 , x2 ): U ’ Rd1 — Rd2 , and metrics gi on Rdi so that gi = x— (¯i ).
¯ ig
He also showed that in this reducible case the holonomy group Hx is a direct product
of the form Hx — Hx where Hx ‚ SO(Vi ). Moreover, it turns out (although this is not
1 2 i

obvious) that, for each of the factor groups Hx , there is a submanifold Mi ‚ M so that
Tx Mi = Vi and so that Hx is the holonomy of the Riemannian metric gi on Mi .
From this discussion it follows that, in order to know which subgroups of SO(n) can
occur as holonomy groups of simply connected Riemannian manifolds, it is enough to ¬nd
the ones that, in addition, act irreducibly on Rn . Using a great deal of machinery from the
theory of representations of Lie groups, M. Berger [Ber] determined a relatively short list

L.8.1 130
of possibilities for irreducible Riemannian holonomy groups. This list was slightly reduced
a few years later, independently by Alexseevski and by Brown and Gray. The result of
their work can be stated as follows:

Theorem 1: Suppose that g is a Riemannian metric on a connected and simply connected
n-manifold M and that the holonomy Hx acts irreducibly on Tx M for some (and hence
every) x ∈ M. Then either (M, g) is locally isometric to an irreducible Riemannian
symmetric space or else there is an isometry ι: Tx M ’ Rn so that H = ι Hx ι’1 is one of
the subgroups of SO(n) in the following table.

Irreducible Holonomies of Non-Symmetric Metrics
Subgroup Conditions Geometrical Type
SO(n) any n generic metric
U(m) n = 2m > 2 K¨hler
SU(m) n = 2m > 2 Ricci-¬‚at K¨hler
Sp(m)Sp(1) n = 4m > 4 Quaternionic K¨hler
Sp(m) n = 4m > 4 hyperK¨hler
G2 n=7 Associative
Spin(7) n=8 Cayley

A few words of explanation and comment about Theorem 1 are in order.
First, a Riemannian symmetric space is a Riemannian manifold di¬eomorphic to a
homogeneous space G/H where H ‚ G is essentially the ¬xed subgroup of an involutory
homomorphism σ: G ’ G that is endowed with a G-invariant metric g that is also invariant
under the involution ι: G/H ’ G/H de¬ned by ι(aH) = σ(a)H. The classi¬cation of the
Riemannian symmetric spaces reduces to a classi¬cation problem in the theory of Lie
algebras and was solved by Cartan. Thus, the Riemannian symmetric spaces may be
regarded as known.
Second, among the holonomies of non-symmetric metrics listed in the table, the ranges
for n have been restricted so as to avoid repetition or triviality. Thus, U(1) = SO(2) and
SU(1) = {e} while Sp(1) = SU(2), and Sp(1)Sp(1) = SO(4).
Third, according to S. T. Yau™s celebrated proof of the Calabi Conjecture, any compact
complex manifold for which the canonical bundle is trivial and that has a K¨hler metric
also has a Ricci-¬‚at K¨hler metric (see [Bes]). For this reason, metrics with holonomy
SU(m) are often referred to as Calabi-Yau metrics.
Finally, I will not attempt to discuss the proof of Theorem 1 in these notes. Even
with modern methods, the proof of this result is non-trivial and, in any case, would take
us far from our present interests. Instead, I will content myself with the remark that it
is now known that every one of these groups does, in fact, occur as the holonomy of a
Riemannian metric on a manifold of the appropriate dimension. I refer the reader to [Bes]
for a complete discussion.

L.8.2 131
We will be particularly interested in the K¨hler and hyperK¨hler cases since these
a a
cases can be characterized by the condition that the holonomy of g leaves invariant certain
closed non-degenerate 2-forms. Hence these cases represent symplectic manifolds with
“extra structure”, namely a compatible metric.

The basic result will be that, for a manifold M that carries one of these two structures,
there is a reduction process that can be applied to suitable group actions on M that preserve
the structure.

K¨hler Manifolds and Algebraic Geometry.
In this section, we give a very brief introduction to K¨hler manifolds. These are


. 24
( 32 .)